The invention refers to a projector lamp, which comprises a light source, a mirror arranged in a space relationship to the light source for projecting the light emitted by the light source in a predetermined direction, the mirror having a surface determined by a carrier element made of glass or plastics, and a layer system covering the surface of the mirror and including layers characterized by refractive indices alternately at least 2.0 and at most 1.7. The projector lamp according to the invention is capable of emitting colour light.
The light sources linked with projector surfaces for projecting white light are well known. Among others the US-A 4 169 238, the DE-A 31 25 267 and DE-A 31 23 670 show projector lamps to be applied in film projector equipments.
If the object is to project colour light and not white, it is known to apply mirrors for reflecting the full range white light and front glass elements, e.g. front lenses for selective reflection or absorption of predetermined light components. The mirrors can be equipped with a thin layer for cooling the light. The disadvantage of applying materials capable of absorbing a required light range, and especially the range of the blue and green light is the high loss of intensity of the light to be projected. The transmissivity of the dicroic front glasses is relatively high (about 80%) but the light sources realised on their basis are relatively expensive because of the necessity of applying a rare mirror and a front element.
The best solution would be to realise a projector system with a mirror for projecting the desired spectrum range onto the surface to be lighted whereby it would be possible to avoid the front filtering elements. As an advantageous solution the thin layers covering the mirror surface should be taken into consideration.
In the first step a difference can be found between the different carrier materials applied for realising the mirror having thin layers on the surface.
The metallic carrier elements generally reflect the ultraviolet, visible and near infrared components of the light, practically the full spectrum range emitted by the light sources.
The glasses--excepting the poor SiO.sub.2 glasses transmitting the three ranges mentioned above--absorb the ultraviolet light and transmit the visible and near infrared components.
The carrier material to be applied in projecting light should show high transmittivity or absorption in the range of the visible light but very low reflectivity in this range. The different glasses and the majority of plastics are applicable, e.g. the policarbonates and the poli-methyl-metacrilate. For preparing lamps having reflection surfaces often a vessel made by pressing from glass or plastics is applied and this basic material can be used as a carrier element.
The next step is to investigate, which kind of changes are required in the surface of the carrier element in order to realise a layer system for reflecting only a selected range of the full spectrum range of the visible light.
From physics it is known that an interference filter can be realised by evaporating or sputtering in vacuum alternately thin layers with low and high refractive indices on the surface of a carrier element.
On the basis of this principle light sources were realised comprising a thin layer system on the outer surface of the vessel, the thin layer exhibiting features of an interference filter. These light sources are capable of emitting colour light. Such arrangements have become known among others from the HU-A 179 695, DE-A 36 17 683 and GB-A 2 103 830.
The afore-mentioned specifications show projector lamps equipped with a front filter and there isn't any known solution whereby projection of colour light would be possible without applying any front filter.